A number of systems have been proposed for performing a vehicle locating function by utilizing an onboard compass and distance measuring device with dead reckoning and automatic position updating. The updating occurs whenever the vehicle passes a signpost that is transmitting its location. These systems are generally disclosed in U.S. Pat. Nos. 3,749,893 and 3,961,166. In actual operation such systems encounter a number of problems. These problems include compass errors caused by the metallic body of the vehicle, permanently magnetized iron in the proximity of the heading sensor and deviations in the earth's magnetic field. Other problems which have been encountered include inaccuracies due to varying wheel diameters, inconvenient map scale units, increased expense due to the high accuracy oscillators required in the update transmitters and receivers, and excessive time required to physically align the heading sensor in the vehicle.
Thus, there is a need for improvements over the existing vehicle locating systems using dead reckoning with proximity signpost updating.